1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory and, more particularly, a method of generating a high voltage at data write.
2. Description of the Related Art
A nonvolatile semiconductor memory such as a flash memory stores logic values of write data as threshold voltages of memory cells. For example, a binary memory cell stores “logic 1” when the threshold voltage is low, and stores “logic 0” when the threshold voltage is high. Generally, operation of decreasing the threshold voltage is referred to as “erase”, and operation of increasing the threshold voltage is referred to as “write” or “program”.
In a NOR-type flash memory, control gates of the memory cells are connected to a word line which is selected corresponding to an address. Drains of the memory cells are connected to a bit line which is selected corresponding to the address. Sources of the memory cells are connected to a common source line.
Write operation of the data requires verify operation for checking the threshold voltage of the memory cell, and program operation for increasing the threshold voltage of the memory cell. In the verify operation, for example, a high voltage (5 V, for example) is supplied to the gates of the memory cells, and a power supply voltage (1.8 V, for example) is supplied to a gate of a transmission transistor connected to the bit lines. A voltage which is lower than the power supply voltage by a threshold voltage of the transmission transistor is supplied to the drains of the memory cells through the transmission transistor. Then, the threshold voltages of the memory cells are checked by currents flowing in the memory cells. That is, the memory cell to be programmed is identified.
In the program operation, a high voltage (9 V, for example) is supplied to the gates of the memory cells, and a high voltage (5.5 V, for example) is supplied to the drains of the memory cells through the transmission transistor. In order to transmit a drain voltage without fail, a high voltage (9 V, for example) is supplied to the gate of the transmission transistor. A charge is then trapped in a charge storing layer of the memory cell, whereby the threshold voltage is increased. Thereafter, the verify operation and the program operation are performed repeatedly until the threshold voltage of each memory cell reaches a desired value.
The above-mentioned flash memory includes a plurality of boosters for generating plural kinds of high voltages. Each booster responds to a write command to start its operation, and responds to the completion of the write operation to stop its operation (Japanese Unexamined Patent Application publication No. 2002-230985, for example).
As described above, the write operation is performed by repeating the verify operation and the program operation. Here, the gate voltage of the memory cells and the gate voltage of the transmission transistor need to be changed significantly between the verify operation and the program operation. Particularly, the gate voltage of the transmission transistor needs to be increased from 1.8 V to 9 V when shifting from the verify operation to the program operation. Boosting time until the booster generates 9 V is long, and write cycle time becomes longer.
Recently, a nonvolatile multilevel semiconductor memory which stores data of plural bits in one memory cell has been developed. In the multilevel memory cell of this kind, the program operation is performed plural times in order to set the threshold voltage accurately (step program method). Since distribution of the threshold voltages becomes narrow in the step program method, a read margin is improved. That is, the memory cell can store the multilevel data without fail. Meanwhile, in the step program method, the verify operation and the program operation are repeated for the plural times, so that one write operation is performed. Since there are a plurality of shifts from the verify operation to the program operation in one write operation, the above-described boosting time has a profound effect thereon.